Heretofore, a tag is disclosed in Japanese Unexamined Patent Publication No. 8-185584, the tag which is constructed such that a resonance circuit block of the tag attached to an object for monitoring a burglar resonates with a specific frequency from a radio wave transmission unit, separation detection means detects whether the tag is separated from the object for monitoring a burglar, and a separation information block controls warning sound output means on the basis of a detection output of this separation detection means. In this burglar-alarm tag, the resonance circuit block is constructed with forming conductive metal foil, having a predetermined shape, by etching and the like in both sides of a thin film of an insulating dielectric. For example, a coil portion formed in a spiral shape with conductive metal foil, and a surface-side planer pattern of a capacitor, connected to the coil portion, in the central portion of the spiral of this coil portion are formed on the surface of the thin film.
In an entrance of an outlet selling the object for monitoring a burglar, a transmission antenna and a reception antenna are installed in a predetermined interval, and these antennas are electrically connected to a control block. The control block is constructed such that the control block makes the transmission antenna transmit a radio wave having a frequency resonating in the resonance circuit block and always checks a signal level of a signal received from the reception antenna. Furthermore, a speaker sounding an alarm is connected to a control output terminal of the control block.
With using a burglar-alarm tag constructed in this manner, a radio wave transmitted from a transmission antenna resonates in a resonance circuit block of the tag attached to an object for monitoring a burglar if an object for monitoring a burglar passes between the transmission antenna and a reception antenna without payment. Hence, the reception antenna receives an input signal whose reception level is modulated. In consequence, the control block sounds an alarm from a speaker and can check takeout of an unpaid product. In regard to a paid-up product, a tag is disabled by breaking a capacitor through applying a strong radio wave, or an alarm is disabled by temporarily stopping the alarm speaker.
Nevertheless, the above burglar-alarm tag requires a transmission antenna and a reception antenna, which are comparatively large, so as to detect a burglar of a product with the tag. Therefore, this type of tag has a defect that a comparatively wide space is required as an installation space of these antennas.
In addition, the above burglar-alarm tag has a possibility of a malfunction in a location with plenty of electrical noise of a computer and the like.
Furthermore, in regard to the above burglar-alarm tag, a central line of the helical coil portion extends in the direction orthogonal to a mounting surface of an object, and hence a radio wave transmitted from the resonance circuit block passes the object. Therefore, if this tag is attached on an object whose surface is made of conductive material such as aluminum or ferromagnetic material such as a steel sheet, self inductance of the coil portion changes. Hence, in comparison with a case that the tag is attached to an object whose surface is made of insulating-material or non-magnetic material, the tag has a possibility of not operating as a burglar-alarm tag due to an influence of a change of a resonance frequency in the resonance circuit block and the like.